Electrolyte formulations for lithium ion batteries

ABSTRACT

Electrolyte formulations including additives or combinations of additives. The electrolyte formulations are useful in lithium ion battery cells having lithium titanate anodes. The electrolyte formulations provide low temperature power performance and high temperature stability in such lithium ion battery cells.

BACKGROUND OF THE INVENTION

The present invention is in the field of battery technology and, moreparticularly, electrolyte formulations that enable both low temperatureand high temperature operation of lithium ion batteries.

Certain applications for lithium ion batteries require wide operatingtemperature ranges. In general, the power capability of lithium ionbatteries suffers at low temperature due to one or more of the followingfactors: 1) an increase in viscosity of the electrolyte resulting inslower lithium ion diffusion; 2) a decrease in the ionic conductivity ofthe electrolyte; 3) a decrease in ionic conductivity of the solidelectrolyte interphase (SEI) on the anode; and 4) a decrease in thediffusion rate of lithium ions through the electrode materials,especially the anode materials.

In the past, solutions to the problems associated with operating alithium ion battery at low temperature have involved adding solventsthat have very low melting points and/or low viscosity to theelectrolyte formulation. Such additional solvents can help prevent theelectrolyte solution from freezing or having substantially increasedviscosity at low temperatures. However, such additional solvents tend tobe detrimental to the high temperature performance of a lithium ionbattery, and in particular the high temperature cycle life.

Certain of the shortcomings of known electrolyte formulations areaddressed by embodiments of the invention disclosed herein by, forexample, improving power performance at low temperature withoutsubstantially decreasing high temperature cycle life. Embodiments hereininclude additives and combinations of additives that improve the powerperformance at low temperature, but improve or maintain the hightemperature cycle life relative to a baseline electrolyte formulation.

BRIEF SUMMARY OF THE INVENTION

Embodiments of the invention include a lithium ion battery cell having afirst electrode, a second electrode formed of lithium titanate and anelectrolyte solution. The electrolyte solution includes additives orcombinations of additives that improve the power performance at lowtemperature, but improve or maintain the high temperature cycle liferelative to baseline electrolyte formulation.

In some embodiments, the electrolyte formulation includes a fluorinatedadditive having chemical structure selected from the group consisting ofcarbonate, borate, oxaborolane, phosphate, phosphonate, phosphazene,ester, and combinations thereof. In some embodiments, the fluorinatedadditive includes a trifluoroethyl group.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 illustrates a schematic of a lithium ion battery implemented inaccordance with an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The following definitions apply to some of the aspects described withrespect to some embodiments of the invention. These definitions maylikewise be expanded upon herein. Each term is further explained andexemplified throughout the description, figures, and examples. Anyinterpretation of the terms in this description should take into accountthe full description, figures, and examples presented herein.

The singular terms “a,” “an,” and “the” include the plural unless thecontext clearly dictates otherwise. Thus, for example, reference to anobject can include multiple objects unless the context clearly dictatesotherwise.

The terms “substantially” and “substantial” refer to a considerabledegree or extent. When used in conjunction with an event orcircumstance, the terms can refer to instances in which the event orcircumstance occurs precisely as well as instances in which the event orcircumstance occurs to a close approximation, such as accounting fortypical tolerance levels or variability of the embodiments describedherein.

The term “about” refers to the range of values approximately near thegiven value in order to account for typical tolerance levels,measurement precision, or other variability of the embodiments describedherein.

A rate “C” refers to either (depending on context) the discharge currentas a fraction or multiple relative to a “1 C” current value under whicha battery (in a substantially fully charged state) would substantiallyfully discharge in one hour, or the charge current as a fraction ormultiple relative to a “1 C” current value under which the battery (in asubstantially fully discharged state) would substantially fully chargein one hour.

To the extent certain battery characteristics can vary with temperature,such characteristics are specified at room temperature (about 25 degreesC.), unless the context clearly dictates otherwise.

Ranges presented herein are inclusive of their endpoints. Thus, forexample, the range 1 to 3 includes the values 1 and 3 as well asintermediate values.

The term “NMC” refers generally to cathode materials containingLiNi_(x)Mn_(y)CO_(z)O_(w), and includes, but is not limited to, cathodematerials containing LiNi_(0.33)Mn_(0.33)Co_(0.33)O₂.

FIG. 1 illustrates a lithium ion battery 100 implemented in accordancewith an embodiment of the invention. The battery 100 includes an anode102, a cathode 106, and a separator 108 that is disposed between theanode 102 and the cathode 106. In the illustrated embodiment, thebattery 100 also includes an electrolyte 104, which is disposed betweenthe anode 102 and the cathode 106 and is formulated to remainsubstantially stable during battery cycling.

The operation of the battery 100 is based upon reversible intercalationand de-intercalation of lithium ions into and from host materials of theanode 102 and the cathode 106. Referring to FIG. 1, the voltage of thebattery 100 is based on redox potentials of the anode 102 and thecathode 106, where Li ions are accommodated or released at a lowerpotential in the former and a higher potential in the latter.

Lithium titanate (e.g., Li₄Ti₅O₁₂; other stoichiometric ratios areincluded in the definition of lithium titanate) (“LTO”) can be used asan active electrode material for an electrode in battery cellapplications that require high power but do not require high energydensity. Batteries with LTO electrodes can operate at a potential ofabout 1.55 V. In many lithium ion batteries using conventionalelectrolyte formulations, components within the electrolyte solutionfacilitate the in-situ formation of a protective film during the initialbattery cycling. This protective film is referred to as a solidelectrolyte interphase (SEI) layer on or next to an anode. The anode SEIcan inhibit further reductive decomposition of the electrolytecomponents. However, it has been observed that SEI formation generallydoes not occur in battery cells with LTO anode. Recalling the factorsabove that are believed to limit low temperature performance ((1) anincrease in viscosity of the electrolyte resulting in slower lithium iondiffusion; (2) a decrease in the ionic conductivity of the electrolyte;(3) a decrease in ionic conductivity of the SEI on the anode; and (4) adecrease in the diffusion rate of lithium ions through the electrodematerials, especially the anode materials), the lack of SEI on an LTOanode means that the electrolyte formulation strongly influence the lowtemperature performance of batteries with LTO anodes.

At high temperature, stability of the battery cell can becomecompromised. Instability at high temperature is believed to be dueto: 1) increased reactivity of electrolyte with an active material; 2)accelerated decomposition of LiPF₆, which generates decompositionproducts that can be reactive with the both the electrolyte and theelectrode active materials; 3) gas generation (primarily H₂) due topresence of aprotic solvents and small amounts of water Parasiticreactions driven by the decomposition products can result in loss ofcell capacity and further decomposition of any SEI.

Referring specifically to battery cells containing an LTO electrode, thehigh temperature stability of the electrolyte formulation can becompromised by catalytic effects of the titanium in certain oxidationstates. At a higher oxidation state, titanium tends to undergo a protonextraction reaction that is believed to be one of main failuremechanisms of LTO anode.

Conventional solutions for the high temperature problem generallyconsist of applying coatings to the surface of the LTO electrodematerial, doping and particle coating. However, such methods tend to beineffective and detrimental to low temperature power performance.

The low temperature performance of cells having LTO anodes is generallybelieved to be limited by the bulk solvent properties. That is, becausethere is no SEI formed on LTO surface to affect the low temperatureperformance, the low temperature performance must be significantlyaffected by the bulk solvent properties. Accordingly, it is expectedthat the addition of an additive would generally increase the cellimpedance, and therefore negatively affect the low temperatureperformance. As a result, little work has been done in the past toinvestigate the effect of additives in an electrolyte formulation inLTO-based batteries.

Low temperature performance in lithium ion batteries can becharacterized by the area specific impedance (ASI), which includescontributions due to the electrode materials, the possible SEI layersformed on those materials, and the bulk electrolyte properties. As thisis a measure of impedance, low ASI values are desirable.

High temperature performance is characterized by measuring the change inASI after storage at elevated temperature. Again, small changes in theASI after storage are desirable, as that would indicate stability of thecell while stored at elevated temperature.

As is described in detail in co-pending application U.S. Ser. No.14/746,746 (Docket #12013US01), which application is incorporated byreference herein in its entirety, electrolyte formulations for widetemperature range performance on LTO anodes must include solvents withgood low temperature properties (low melting point, low viscosity, highconductivity, etc.). Additives that can form conductive and robustprotection layer on LTO surface to not only improve interfacial ionicconductivity but also mitigate catalytic reactivity of Ti³⁺/Ti⁴⁺especially at elevated temperatures.

In certain embodiments, the addition of a single additive compoundimproves the low temperature power performance of batteries having LTOanodes. For example, tris(2,2,2-trifluoroethyl)borate (structure (a)):

improves low temperature power performance. Another additive compound,tris(2,2,2-trifluoroethyl)phosphate (structure (b)):

also improves low temperature power performance. Still another additivecompound, methyl 2,2,2,-trifluoroethyl carbonate (structure (c)):

improves low temperature power performance.

The low temperature power performance of batteries having LTO anodes andcontaining electrolyte formulations including these additives ispresented below in Table 2.

In certain embodiments, the addition of a single additive compoundimproves the high temperature stability of batteries having LTO anodes.For example, tris(2,2,2-trifluoroethyl)borate (structure (a) above)improves high temperature stability. Also,4,4,5,5-tetramethyl-2-(4-trifluoromethylphenyl)-1,3,2-dioxaborolane(structure (d)):

improves high temperature stability. Ethyl difluoroacetate (structure(e)):

also improves high temperature stability. Another additive compound,diethyl (difluoromethyl)phosphonate (structure (f)):

improves high temperature stability. The additive compoundhexakis(1H,1H-trifluoroethoxy)phosphazene (structure (g)):

also improves high temperature stability. Still another additivecompound, bis(2,2,2-trifluoroethyl)carbonate (structure (h)):

improves high temperature stability.

In certain embodiments of additive combinations disclosed herein, theelectrolyte formulation includes certain boron-containing additives. Theboron-containing additives are often strong electrophiles. In otherwords, they readily react with reductive decomposition intermediatesfrom solvents and salts on the anode, which may result in a thinner butmore thermally stable SEI. Effective boron-containing additives arebelieved to be highly activated compounds that contain at least oneactivated B—O bond.

In some embodiments, the boron-containing additive is a compoundrepresented by structural formula (i):

where at least one of R₁, R₂ and R₃ includes a fluorine. R₁, R₂ and areindependently selected from the group consisting of substituted C₁-C₂₀alkyl groups, substituted C₁-C₂₀ alkenyl groups, substituted C₁-C₂₀alkynyl groups, and substituted C₅-C₂₀ aryl groups. At least one of thesubstitutions is a fluorine, and other additional substitutions arepossible, include further fluorine substitutions. Preferred embodimentsinclude tris(2,2,2-trifluoroethyl)borate and its derivatives.

In some embodiments, the boron-containing additive is a compoundrepresented by structural formula (j):

where R includes at least one electron-withdrawing moiety. Examples ofelectron withdrawing moieties include fluorine atoms, certain fluorinesubstituted structures, and structures having unsaturated carbons.Preferred embodiments include certain oxaborinanes and oxaborolanes.Preferred embodiments include4,4,5,5-tetramethyl-2-(4-trifluoromethylphenyl)-1,3,2-dioxaborolane andits derivatives.

The high temperature stability of batteries having LTO anodes andcontaining electrolyte formulations including these additives ispresented below in Table 2.

In certain embodiments, combinations of additives improve the wideoperating temperature performance of lithium ion batteries having LTOanodes. Additive combinations were tested based on the improvementsobserved for the electrolyte formulations includes a single additive.For example, if additive A improves low temperature properties whileadditive B and additive C only improve high temperature properties,additive A would then be tested in combination with additive B and incombination with additive C. Notably, combining an additive shown toimprove low temperature power performance with an additive shown toimprove high temperature stability does not necessarily result in aformulation with improved low and high temperature properties. Thecombinations sometimes perform synergistically and sometimes do not.

A set of three low temperature additives were chosen to be combined witha set of five high temperature additives. The three low temperaturepower performance additives were tris(2,2,2-trifluoroethyl)borate(“TTFEB”), tris(2,2,2-trifluoroethyl)phosphate (“TTFEP”) and methyl2,2,2,-trifluoroethyl carbonate (“MTFEC”). The five high temperaturestability additives were TTFEB,4,4,5,5-tetramethyl-2-(4-trifluoromethylphenyl)-1,3,2-dioxaborolane(“TFMPDB”), ethyl difluoroacetate (“EDFA”), diethyl(difluoromethyl)phosphonate (“DFMP”), and lithium bis(oxalato)borate(“LiBOB”). In this case, LiBOB was used as a control. Thus, there were atotal of 14 combinations as shown in Table 1 below. In general,additives were combined at their optimal concentration as determined bysingle additive testing.

TABLE 1 Summary of Additive Combinations 2% TTFEB 0.5% TTFEP 2% MTFEC0.5% LiBOB X X X (Control) 0.5% TTFEB X X 0.5% TFMPDB X X X 0.5% DFMP XX X 2% EDFA X X X

The following examples describe specific aspects of some embodiments ofthe invention to illustrate and provide a description for those ofordinary skill in the art. The examples should not be construed aslimiting the invention, as the examples merely provide specificmethodology useful in understanding and practicing some embodiments ofthe invention.

Examples Electrolyte Solution Formulation

Electrolyte formulas included a lithium salt and a solvent blend. Thelithium salt was LiPF₆, and was used at a concentration of 1.2M. Solventblends were formulated from propylene carbonate (PC), sulfolane (SL),ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), diethylcarbonate (DEC), methyl butyrate (MB) and methyl acetate (MA). Sevendifferent solvent blends formulations were used:

Solvent Blend 1: PC/EMC/DMC/MB (20/30/40/10 by volume)

Solvent Blend 2: SL/EMC/DMC/MB (20/30/40/10 by volume)

Solvent Blend 3: PC/SL/EMC/DMC/MA (5/15/30/40/10 by volume)

Solvent Blend 4: PC/SL/EMC/DMC/MB (12.5/12.5/28.1/37.5/9.4 by volume)

Solvent Blend 5: SL/EMC/DMC/MA (25/28.1/37.5/9.4 by volume)

Solvent Blend 6: SL/EMC/DMC/DEC (25/28.1/37.5/9.4 by volume)

Solvent Blend 7: PC/EMC/DMC/MB (33.3/25/33.4/8.3 by volume)

Additives were included at concentrations varying between 0.5% and 2.0%by weight. A control electrolyte containing no additives was also used.

Battery Assembly.

Battery cells were formed in a high purity argon filled glove box(M-Braun, O₂ and humidity content <0.1 ppm). A LiNi_(x)Mn_(y)Co_(z)O₂(NMC, X+Y+Z=1) cathode material and a lithium titanate (LTO) anodematerial were used. Each battery cell includes the composite cathodefilm, a polyolefin separator, and the composite anode film. Electrolyteformulations were made according to the ratios and components describedherein and added to the battery cell.

Electrochemical Formation.

The formation cycle for these NMC/LTO battery cells was a 6 hour opencircuit voltage (OCV) hold followed by a charge to 2.8 V at rate C/10,with a constant voltage (CV) hold to C/20. The formation cycle wascompleted with a C/10 discharge to 1.5 V. All formation cycles were runat room temperature.

Electrochemical Characterization.

Initial area specific impedance (ASI) was measured after setting thetarget state of charge (SOC) by discharging the cell at rate of C/10 andthen applying a 10 second pulse at a rate of 5 C. Low temperature ASIresults were derived as follows: The cell was recharged to 2.8 V at arate of C/5 at room temperature, with a CV hold at C/10 followed by aone hour OCV hold. Then, the ambient temperature was reduced to −25degrees Celsius, followed by a 12 hour OCV hold to allow the test systemtemperature to equilibrate. All discharges to the specified SOC whereconducted at −25 degrees Celsius at a rate of C/10, with a one-hour restat the specified SOC. A discharge pulse at 50% SOC was done at a rate of2 C for 10 seconds, followed by a 40 second rest. ASI was calculatedfrom the initial voltage (V) prior to the pulse and the final voltage(V_(f)) at the end of the pulse according to Formula (1), where A is thecathode area and i is the current:

$\begin{matrix}{{{ASI}( {\Omega \cdot {cm}^{2}} )} = \frac{( {V_{i} - V_{f}} ) \times A}{i}} & (1)\end{matrix}$

After full recharge to 2.8 V at room temperature, the cells were thenstored at 60 degrees Celsius at OCV for two weeks. After two weeks thecells were removed from high temperature storage and then allowed toequilibrate to room temperature. The ASI was then measured by the sameprotocol used to determine initial ASI (setting the target SOC, and thenapplying a 10 second pulse at a rate of 5 C).

Results

The following tables present the results of the testing described hereinof certain embodiments of the invention. The tables below identity theadditive or the additive combination tested, with the concentration(described as a weight percent of the total formulation) in parentheses.In the case of additive combinations, the solvent blend is alsoidentified. The tables also present the discharge capacity (in units ofmAh/cm²) measured at the first cycle and the coulombic efficiency (as apercent) of the first cycle. To demonstrate the wide operatingtemperature performance, several ASI measurements are listed in thetables. The column labeled “−25 C ASI” presents the data collected fromthe low temperature measurements of ASI (in units of Ω*cm²). The columnlabeled “1st ASI” presents the data collected from the initial roomtemperature measurements of ASI (in units of Ω*cm²). The column labeled“2nd ASI” presents the data collected from the measurements of ASI afterhigh temperature storage (in units of Ω*cm²). The column labeled “DeltaASI” is the difference between the 1st ASI data and the 2nd ASI data.Thus, numbers lower than control for −25 C ASI and Delta ASI demonstrateimprovements in low power performance and high temperature stability,respectively. Further, for wide operating temperature performance it ispreferred that the values for 1st ASI be less than or equal to the 1stASI value of the control.

Table 2 presents the data from testing of single additives in theelectrolyte formulation PC/EMC/DMC/MB (20/30/40/10 by volume), 1.2MLiPF₆. The additive tris(2,2,2-trifluoroethyl) borate (TTFEB) at 2.0weight percent demonstrates the largest improvement in low temperaturepower performance, while methyl 2,2,2,-trifluoroethyl carbonate (MTFEC)at 2.0 weight percent and tris(2,2,2-trifluoroethyl)phosphate (TTFEP) at0.5 weight percent also demonstrate improvement in low temperature powerperformance.

Still referring to Table 2, diethyl(difluoromethyl)phosphonate (DFMP) at0.5 weight percent demonstrates the largest improvement in hightemperature stability. The additives tris(2,2,2-trifluoroethyl)borate(TTFEB) at 0.5 weight percent,4,4,5,5-tetramethyl-2-(4-trifluoromethylphenyl)-1,3,2-dioxaborolane(TFMPDB) at 0.5 weight percent, and ethyl difluoroacetate (EDFA) at 2.0weight percent demonstrate 1st ASI, 2nd ASI and Delta ASI values thatare improved as compared to the control values.

TABLE 2 Summary of additives in solvent blend 1 Cyc1 Cyc1 −25 C.Additive Capacity CE ASI 1st 2nd Delta (wt %) (mAh/cm²) (%) (Ω*cm²) ASIASI ASI Control (0) 1.0 89.8 138.5 15.8 26.5 10.7 tris(2,2,2- 1.0 92.2109.3 17.1 27.8 10.7 trifluoroethyl) borate (2.0) methyl 2,2,2,- 1.090.8 127.4 17.0 32.8 15.8 trifluoroethyl carbonate (2.0) tris(2,2,2- 1.090.2 128.7 16.3 33.9 17.6 trifluoroethyl) phosphate (0.5) methyl 2,2,2,-1.0 88.3 133.4 19.0 39.6 20.6 trifluoroethyl carbonate (0.5) diethyl 1.090.7 155.5 15.4 19.1 3.7 (difluoromethyl) phosphonate (0.5) tris(2,2,2-1.0 92.2 122.8 15.2 22.5 7.3 trifluoroethyl) borate (0.5) hexakis(1H,1H-1.0 90.2 186.4 17.2 24.7 7.5 trifluoroethoxy) phosphazene (2.0)bis(2,2,2- 1.0 89.8 164.9 16.8 25.1 8.3 trifluoroethyl) carbonate (2.0)4,4,5,5- 1.0 91.4 141.0 14.2 23.0 8.9 tetramethyl-2- (4-trifluoromethylphenyl)-1,3,2- dioxaborolane (0.5) Ethyl 1.0 90.6 137.0 14.0 23.1 9.1difluoroacetate (2.0)

Table 3 presents the data from testing of additive combinations in theelectrolyte formulation PC/EMC/DMC/MB (20/30/40/10 by volume), 1.2MLiPF₆. Several combinations demonstrated improved wide operatingtemperature range performance as compared to the control, including 2.0weight percent MTFEC with 0.5 weight percent TTFEB, 2.0 weight percentMTFEC with 0.5 weight percent DFMP, 0.5 weight percent TTFEP with 0.5weight percent TTFEB, 0.5 weight percent TTFEP with 0.5 weight percentDFMP, 2.0 weight percent TTFEB with 0.5 weight percent TFMPDB, and 2.0weight percent TTFEB with 0.5 weight percent DFMP. The combination of2.0 weight percent MTFEC with 0.5 weight percent LiBOB also showedimproved wide operating temperature range performance.

TABLE 3 Summary of additive combinations in solvent blend 1 AdditiveCyc1 Cyc1 −25 C. combination Capacity CE ASI 1st 2nd Delta (wt %)(mAh/cm²) (%) (Ω*cm²) ASI ASI ASI Solvent Blend 1.0 89.8 138.5 15.8 26.510.7 1 with no additive MTFEC (2.0)/ 1.0 89.9 131.7 13.0 18.5 5.5 TTFEB(0.5) MTFEC (2.0 / 1.0 91.3 142.5 13.0 19.1 6.1 TFMPDB (0.5) MTFEC(2.0)/ 1.0 91.1 114.2 13.5 18.3 4.8 DFMP (0.5) MTFEC (2.0)/ 1.1 80.7310.3 13.8 20.1 6.3 EDFA (2.0) MTFEC (2.0)/ 1.0 91.2 119.0 13.8 20.9 7.1LiBOB (0.5) TTFEP (0.5)/ 1.0 92.3 117.5 13.7 20.7 7.0 TTFEB (0.5) TTFEP(0.5)/ 1.0 89.8 156.6 12.6 19.6 7.0 TFMPDB (0.5) TTFEP (0.5)/ 1.0 91.8115.6 13.0 18.0 5.0 DFMP (0.5) TTFEP (0.5)/ 1.0 90.3 157.1 15.6 25.910.2 EDFA (2.0) TTFEP (0.5)/ 1.0 91.4 152.6 13.8 19.6 5.7 LiBOB (0.5)TTFEB (2.0)/ 1.0 89.6 102.5 13.7 17.8 4.1 TFMPDB (0.5) TTFEB (2.0)/ 1.092.1 101.0 13.4 17.3 3.8 DFMP (0.5) TTFEB (2.0)/ 1.0 92.2 97.4 15.2 27.111.9 EDFA (2.0) TTFEB (2.0)/ 1.0 87.3 144.9 16.3 24.4 8.1 LiBOB (0.5)

Table 4 presents the data from testing of additive combinations in theelectrolyte formulation SL/EMC/DMC/MB (20/30/40/10 by volume), 1.2MLiPF₆. Several of the additive combinations provided improved lowtemperature power performance as compared to control and some additivecombinations provided improved high temperature stability as compared tocontrol. For example, 2.0 weight percent TTFEB with 0.5 weight percentTFMPDB showed improved performance.

TABLE 4 Summary of additive combinations in solvent blend 2 AdditiveCyc1 Cyc1 −25 C. combination Capacity CE ASI 1st 2nd Delta (wt %)(mAh/cm²) (%) (Ω*cm²) ASI ASI ASI Solvent Blend 1.0 92.4 101.3 15.3 20.35.0 2 with no additive MTFEC (2.0)/ 1.0 91.9 110.0 14.7 19.9 5.3 TTFEB(0.5) MTFEC (2.0)/ 1.0 91.4 118.2 15.6 22.6 7.0 TFMPDB (0.5) MTFEC(2.0)/ 1.0 91.0 109.4 13.2 18.7 5.5 DFMP (0.5) MTFEC (2.0)/ 1.0 90.198.9 13.2 26.4 13.2 EDFA (2.0) MTFEC (2.0)/ 1.0 91.1 116.4 13.9 20.4 6.5LiBOB (0.5) TTFEP (0.5)/ 1.0 88.0 122.5 16.2 20.9 4.7 TTFEB (0.5) TTFEP(0.5)/ 1.0 91.5 126.8 16.5 21.7 5.2 TFMPDB (0.5) TTFEP (0.5)/ 1.0 91.7128.1 15.1 17.8 2.7 DFMP (0.5) TTFEP (0.5)/ 1.0 91.3 127.9 15.3 21.1 5.8EDFA (2.0) TTFEP (0.5)/ 1.0 90.6 114.4 16.1 23.6 7.5 LiBOB (0.5) TTFEB(2.0)/ 1.0 91.9 99.9 14.2 18.8 4.6 TFMPDB (0.5) TTFEB (2.0)/ 1.0 84.5128.4 15.7 18.7 3.1 DFMP (0.5) TTFEB (2.0)/ 1.0 86.3 128.5 15.9 24.3 8.4EDFA (2.0) TTFEB (2.0)/ 1.1 79.1 129.6 15.8 19.3 3.5 LiBOB (0.5)

Table 5 presents the data from testing of additive combinations in theelectrolyte formulation PC/SL/EMC/DMC/MA (5/15/30/40/10 by volume), 1.2MLiPF₆. Several of the additive combinations provided improved lowtemperature power performance as compared to control and some additivecombinations provided improved high temperature stability as compared tocontrol. For example, 2.0 weight percent MTFEC with 0.5 weight percentTTFEB, 2.0 weight percent MTFEC with 0.5 weight percent DFMP, 0.5 weightpercent TTFEP with 0.5 weight percent DFMP, and 0.5 weight percent TTFEPwith 0.5 weight percent LiBOB showed improved performance.

TABLE 5 Summary of additive combinations in solvent blend 3 AdditiveCyc1 Cyc1 −25 C. combination Capacity CE ASI 1st 2nd Delta (wt %)(mAh/cm²) (%) (Ω*cm²) ASI ASI ASI Solvent Blend 1.0 90.6 130.1 12.5 16.03.9 3 with no additive MTFEC (2.0)/ 1.0 91.9 102.5 13.9 15.7 2.4 TTFEB(0.5) MTFEC (2.0)/ 1.0 90.8 111.9 14.6 21.0 6.7 TFMPDB (0.5) MTFEC(2.0)/ 1.0 92.1 126.3 13.6 15.5 2.3 DFMP (0.5) MTFEC (2.0)/ 1.0 90.4124.8 14.2 18.3 4.6 EDFA (2.0) MTFEC (2.0)/ 1.0 91.2 133.9 13.9 18.0 4.3LiBOB (0.5) TTFEP (0.5)/ 1.0 92.3 109.9 17.0 21.3 4.8 TTFEB (0.5) TTFEP(0.5)/ 1.0 89.6 124.9 14.7 21.9 7.6 TFMPDB (0.5) TTFEP (0.5)/ 1.0 91.7126.5 14.0 16.1 2.5 DFMP (0.5) TTFEP (0.5)/ 1.0 90.3 120.5 15.1 20.6 6.0EDFA (2.0) TTFEP (0.5)/ 1.0 91.6 114.4 14.5 17.9 3.6 LiBOB (0.5) TTFEB(2.0)/ 1.0 92.6 90.8 13.5 19.1 6.2 TFMPDB (0.5) TTFEB (2.0)/ 1.0 92.490.2 13.1 17.0 4.5 DFMP (0.5) TTFEB (2.0)/ 1.0 92.3 87.4 13.3 21.3 8.6EDFA (2.0) TTFEB (2.0)/ 1.0 88.5 107.9 13.9 21.6 8.1 LiBOB (0.5)

Table 6 presents the data from testing of additive combinations in theelectrolyte formulation PC/SL/EMC/DMC/MB (12.5/12.5/28.1/37.5/9.4 byvolume), 1.2M LiPF₆. Several of the additive combinations providedimproved low temperature power performance as compared to control andsome additive combinations provided improved high temperature stabilityas compared to control. For example, 2.0 weight percent TTFEB with 0.5weight percent TFMPDB showed improved performance.

TABLE 6 Summary of additive combinations in solvent blend 4 AdditiveCyc1 Cyc1 −25 C. combination Capacity CE ASI 1st 2nd Delta (wt %)(mAh/cm²) (%) (Ω*cm²) ASI ASI ASI Solvent Blend 1.0 91.7 98.7 12.4 18.25.7 4 with no additive MTFEC (2.0)/ 1.0 90.2 111.6 15.5 30.0 14.6 TTFEB(0.5) MTFEC (2.0)/ 1.0 90.5 114.7 13.3 18.1 4.8 TFMPDB (0.5) MTFEC(2.0)/ 1.0 90.0 127.0 15.9 36.7 20.7 DFMP (0.5) MTFEC (2.0)/ 1.0 92.0140.4 16.8 25.0 8.2 EDFA (2.0) MTFEC (2.0)/ 1.0 92.7 116.8 16.1 23.1 7.0LiBOB (0.5) TTFEP (0.5)/ 1.0 90.8 134.3 14.3 21.5 7.2 TTFEB (0.5) TTFEP(0.5)/ 1.0 92.4 147.0 16.2 18.3 2.1 TFMPDB (0.5) TTFEP (0.5)/ 1.0 92.3131.1 13.4 20.0 6.6 DFMP (0.5) TTFEP (0.5)/ 1.0 92.4 152.3 17.5 24.4 6.9EDFA (2.0) TTFEP (0.5)/ 1.0 92.8 108.9 14.4 20.0 5.6 LiBOB (0.5) TTFEB(2.0)/ 1.0 93.0 97.8 12.1 15.6 3.5 TFMPDB (0.5) TTFEB (2.0)/ 1.0 92.9122.4 15.5 25.4 9.9 DFMP (0.5) TTFEB (2.0)/ 1.0 85.5 154.9 16.0 25.9 9.9EDFA (2.0) TTFEB (2.0)/ 1.0 91.7 98.7 12.4 18.2 5.7 LiBOB (0.5)

Table 7 presents the data from testing of additive combinations in theelectrolyte formulation SL/EMC/DMC/MA (25/28.1/37.5/9.4 by volume), 1.2MLiPF₆. Several of the additive combinations provided improved lowtemperature power performance as compared to control. For example, 0.5weight percent TTFEP with 0.5 weight percent TTFEB and 2.0 weightpercent TTFEB with 0.5 weight percent DFMP showed improved performance.

TABLE 7 Summary of additive combinations in solvent blend 5 AdditiveCyc1 Cyc1 −25 C. combination Capacity CE ASI 1st 2nd Delta (wt %)(mAh/cm²) (%) (Ω*cm²) ASI ASI ASI Solvent Blend 1.0 92.5 115.6 14.0 18.34.3 5 with no additive MTFEC (2.0)/ 1.0 90.5 101.0 15.6 23.2 7.7 TTFEB(0.5) MTFEC (2.0)/ 1.0 88.2 112.8 14.6 31.0 16.4 TFMPDB (0.5) MTFEC(2.0)/ 1.0 91.0 116.8 15.8 26.4 10.6 DFMP (0.5) MTFEC (2.0)/ 1.0 89.7110.9 14.4 30.7 16.3 EDFA (2.0) MTFEC (2.0)/ 1.0 91.5 107.9 16.3 23.06.7 LiBOB (0.5) TTFEP (0.5)/ 1.0 92.1 89.7 14.0 18.4 4.4 TTFEB (0.5)TTFEP (0.5)/ 1.0 92.0 139.0 16.3 24.0 7.6 TFMPDB (0.5) TTFEP (0.5)/ 1.092.0 126.9 15.9 21.4 5.5 DFMP (0.5) TTFEP (0.5)/ 1.0 91.8 130.1 16.523.8 7.3 EDFA (2.0) TTFEP (0.5)/ 1.0 91.3 153.9 18.4 22.9 4.5 LiBOB(0.5) TTFEB (2.0)/ 1.0 92.8 109.7 14.8 20.8 5.9 TFMPDB (0.5) TTFEB(2.0)/ 1.0 92.5 86.5 13.8 18.1 4.3 DFMP (0.5) TTFEB (2.0)/ 1.0 92.5113.0 15.6 27.0 11.4 EDFA (2.0) TTFEB (2.0)/ 1.0 87.9 105.0 15.0 23.07.9 LiBOB (0.5)

Table 8 presents the data from testing of additive combinations in theelectrolyte formulation SL/EMC/DMC/DEC (25/28.1/37.5/9.4 by volume),1.2M LiPF₆. Some of the additive combinations provided improved lowtemperature power performance as compared to control.

TABLE 8 Summary of additive combinations in solvent blend 6 AdditiveCyc1 Cyc1 −25 C. combination Capacity CE ASI 1st 2nd Delta (wt %)(mAh/cm²) (%) (Ω*cm²) ASI ASI ASI Solvent Blend 1.0 91.8 105.1 14.9 18.13.3 6 with no additive MTFEC (2.0)/ 1.0 91.8 125.6 22.2 36.3 14.1 TTFEB(0.5) MTFEC (2.0)/ 1.0 91.8 129.0 15.7 28.8 13.1 TFMPDB (0.5) MTFEC(2.0)/ 1.0 89.3 120.1 17.6 27.9 10.3 DFMP (0.5) MTFEC (2.0)/ 1.0 89.2120.9 17.5 120.7 103.2 EDFA (2.0) MTFEC (2.0)/ 1.0 91.4 147.1 17.4 24.47.0 LiBOB (0.5) TTFEP (0.5)/ 1.0 92.5 102.4 15.2 21.2 6.1 TTFEB (0.5)TTFEP (0.5)/ 1.0 92.5 131.5 15.4 23.0 7.6 TFMPDB (0.5) TTFEP (0.5)/ 1.092.2 125.2 16.1 22.0 5.9 DFMP (0.5) TTFEP (0.5)/ 1.0 91.8 115.4 14.724.1 9.4 EDFA (2.0) TTFEP (0.5)/ 1.0 92.1 131.8 14.9 19.6 4.7 LiBOB(0.5) TTFEB (2.0)/ 1.0 92.9 116.8 15.7 21.3 5.6 TFMPDB (0.5) TTFEB(2.0)/ 1.0 92.2 96.2 16.4 24.3 7.8 DFMP (0.5) TTFEB (2.0)/ 1.0 92.3 89.816.6 31.5 15.0 EDFA (2.0) TTFEB (2.0)/ 1.0 88.3 113.8 18.0 33.0 15.0LiBOB (0.5)

Table 9 presents the data from testing of additive combinations in theelectrolyte formulation PC/EMC/DMC/MB (33.3/25/33.4/8.3 by volume), 1.2MLiPF₆. The additive combinations of 0.5 weight percent TTFEP with 0.5weight percent TTFEB, 0.5 weight percent TTFEP with 0.5 weight percentDFMP, 0.5 weight percent TTFEP with 0.5 weight percent LiBOB, and 2.0weight percent TTFEB with 0.5 weight percent DFMP demonstrated improvedwide operating temperature range performance as compared to the control.

TABLE 9 Summary of additive combinations in solvent blend 7 AdditiveCyc1 Cyc1 −25 C. combination Capacity CE ASI 1st 2nd Delta (wt %)(mAh/cm²) (%) (Ω*cm²) ASI ASI ASI Solvent Blend 1.0 91.5 148.5 15.5 21.25.7 7 with no additive MTFEC (2.0)/ 1.0 91.1 116.5 13.6 20.1 6.6 TTFEB(0.5) MTFEC (2.0)/ 1.0 90.4 106.8 14.0 20.4 6.4 TFMPDB (0.5) MTFEC(2.0)/ 1.0 88.1 129.7 15.6 25.2 9.6 DFMP (0.5) MTFEC (2.0)/ 1.0 85.5118.6 14.2 27.7 13.5 EDFA (2.0) MTFEC (2.0)/ 1.0 91.7 150.0 15.9 24.98.9 LiBOB (0.5) TTFEP (0.5)/ 1.0 91.8 110.6 13.8 18.6 4.8 TTFEB (0.5)TTFEP (0.5)/ 1.0 89.6 123.5 15.2 23.1 7.9 TFMPDB (0.5) TTFEP (0.5)/ 1.089.7 135.8 13.7 19.0 5.2 DFMP (0.5) TTFEP (0.5)/ 1.0 88.3 116.5 13.122.0 8.9 EDFA (2.0) TTFEP (0.5)/ 1.0 90.5 127.4 13.9 18.4 4.5 LiBOB(0.5) TTFEB (2.0)/ 1.0 92.9 107.3 14.0 21.2 7.1 TFMPDB (0.5) TTFEB(2.0)/ 1.0 92.3 100.5 12.9 16.1 3.2 DFMP (0.5) TTFEB (2.0)/ 1.0 92.3100.3 14.1 22.1 8.0 EDFA (2.0) TTFEB (2.0)/ 1.0 87.8 129.1 14.7 23.1 8.4LiBOB (0.5)

For all of the additive and additive combinations that provided improvedlow temperature power performance, high temperature stability, or both,no negative effects on initial discharge capacities or coulombicefficiencies were observed as compared to the control electrolyteformulations.

Without being bound to a particular hypothesis, theory, or proposedmechanism of action, the performance improvement imparted by theadditives or combinations of additives is due to improvements in the SEIlayer, specifically on the LTO anode. LTO anodes operate at a muchhigher voltage than graphite anodes. At these higher voltages,conventional additives used to produce SEI on graphite anodes cannot bereduced to form a passivation layer on an LTO anode. However, thechemical reduction potential at the electrode/electrolyte interface canbe significantly increased in presence of strong electron-withdrawingfunctionality. In embodiments disclosed herein, the fluorinated groupsin the additives provide that strong electron-withdrawing functionality,which allows the additives to function as SEI forming additives toimprove the low temperature power performance and/or high temperaturestability of LTO anodes. However, it is important to note that certaincombinations of fluorinated additives provide low temperature powerperformance, high temperature stability, or both. It is not obviouswhich combinations will provide wide operating temperature rangeperformance.

Further, the additives and an additive compounds disclosed herein mayprovide for the formation on an electrochemically active SEI that isformed due to the specific chemical interactions between these additivesand the LTO anode. That is, the reaction products formed by theseadditives and the LTO surface may facilitate the formation of anelectrochemically active SEI. This is a surprising result given that anSEI formed on an LTO surface would be expected to increase the impedanceof the electrochemical cell, and indeed that increase in impedance isseen in certain additives combinations in certain solvent blends foundin the tables above.

The fluorinated chemical structures that have demonstrated improved lowtemperature power performance, high temperature stability, or bothinclude carbonates, borates, oxaborolanes, phosphates, phosphonates,phosphazene, and esters. It is anticipated, based on the disclosuressupported by the testing herein, that certain fluorinated version ofthese chemical structures will provide low temperature powerperformance, high temperature stability, or both. Indeed, it isanticipated that other strong electron-withdrawing functionality may becombined with the chemical structures disclosed herein (e.g.,carbonates, borates, oxaborolanes, phosphates, phosphonates,phosphazene, and esters) to yield additive compounds that alone or incombination will provide low temperature power performance, hightemperature stability, or both.

While the invention has been described with reference to the specificembodiments thereof, it should be understood by those skilled in the artthat various changes may be made and equivalents may be substitutedwithout departing from the true spirit and scope of the invention asdefined by the appended claims. In addition, many modifications may bemade to adapt a particular situation, material, composition of matter,method, or process to the objective, spirit and scope of the invention.All such modifications are intended to be within the scope of the claimsappended hereto. In particular, while the methods disclosed herein havebeen described with reference to particular operations performed in aparticular order, it will be understood that these operations may becombined, sub-divided, or re-ordered to form an equivalent methodwithout departing from the teachings of the invention. Accordingly,unless specifically indicated herein, the order and grouping of theoperations are not limitations of the invention.

1. A lithium ion battery cell, comprising: a first electrode; a secondelectrode comprising a lithium titanate active material; and anelectrolyte formulation comprising LiPF₆ and comprising a firstadditive, wherein the first additive comprisestris(2,2,2-trifluoroethyl)borate, and a second additive, wherein thesecond additive comprises a fluorinated compound other thantris(2,2,2-trifluoroethyl)borate.
 2. The lithium ion battery cell ofclaim 1, wherein the second additive comprises a fluorinated borate. 3.The lithium ion battery cell of claim 1, wherein the second additivecomprises a fluorinated oxaborolane.
 4. The lithium ion battery cell ofclaim 3, wherein the second additive comprises4,4,5,5-tetramethyl-2-(4-trifluoromethylphenyl)-1,3,2-dioxaborolane. 5.The lithium ion battery cell of claim 1, wherein the second additivecomprises a fluorinated ester.
 6. The lithium ion battery cell of claim5, wherein the second additive comprises ethyl difluoroacetate.
 7. Thelithium ion battery cell of claim 1, wherein the second additivecomprises a fluorinated phosphonate.
 8. The lithium ion battery cell ofclaim 7, wherein the second additive comprises diethyl(difluoromethyl)phosphonate.
 9. The lithium ion battery cell of claim 1,wherein the second additive comprises a fluorinated phosphate.
 10. Thelithium ion battery cell of claim 9, wherein the second additivecomprises tris(2,2,2-trifluoroethyl)phosphate.
 11. The lithium ionbattery cell of claim 1, wherein the second additive comprises afluorinated carbonate.
 12. The lithium ion battery cell of claim 11,wherein the second additive comprises methyl 2,2,2-trifluoroethylcarbonate.
 13. A lithium ion battery cell, comprising: a firstelectrode; a second electrode comprising a lithium titanate activematerial; and an electrolyte formulation comprising LiPF₆ and comprisinga first additive, wherein the first additive comprisestris(2,2,2-trifluoroethyl)phosphate, and a second additive, wherein thesecond additive comprises a fluorinated compound other thantris(2,2,2-trifluoroethyl)phosphate.
 14. The lithium ion battery cell ofclaim 13, wherein the second additive comprises a fluorinated borate.15. The lithium ion battery cell of claim 1, wherein the second additivecomprises a fluorinated oxaborolane.
 16. The lithium ion battery cell ofclaim 15, wherein the second additive comprises4,4,5,5-tetramethyl-2-(4-trifluoromethylphenyl)-1,3,2-dioxaborolane. 17.The lithium ion battery cell of claim 13, wherein the second additivecomprises a fluorinated ester.
 18. The lithium ion battery cell of claim17, wherein the second additive comprises ethyl difluoroacetate.
 19. Thelithium ion battery cell of claim 13, wherein the second additivecomprises a fluorinated phosphonate.
 20. The lithium ion battery cell ofclaim 19, wherein the second additive comprises diethyl(difluoromethyl)phosphonate.
 21. A lithium ion battery cell, comprising:a first electrode; a second electrode comprising a lithium titanateactive material; and an electrolyte formulation comprising LiPF₆ andcomprising a first additive, wherein the first additive comprises methyl2,2,2-trifluoroethyl carbonate, and a second additive, wherein thesecond additive comprises a fluorinated compound other than methyl2,2,2-trifluoroethyl carbonate.
 22. The lithium ion battery cell ofclaim 21, wherein the second additive comprises a fluorinated borate.23. The lithium ion battery cell of claim 21, wherein the secondadditive comprises a fluorinated oxaborolane.
 24. The lithium ionbattery cell of claim 23, wherein the second additive comprises4,4,5,5-tetramethyl-2-(4-trifluoromethylphenyl)-1,3,2-dioxaborolane. 25.The lithium ion battery cell of claim 21, wherein the second additivecomprises a fluorinated ester.
 26. The lithium ion battery cell of claim25, wherein the second additive comprises ethyl difluoroacetate.
 27. Thelithium ion battery cell of claim 21, wherein the second additivecomprises a fluorinated phosphonate.
 28. The lithium ion battery cell ofclaim 27, wherein the second additive comprises diethyl(difluoromethyl)phosphonate.